The present invention relates to one use of a transfer belt for a soft tissue paper machine.
DE-195 48 747 discloses a paper machine for making creped tissue, which has a press comprising a shoe press roll, a counter roll and a suction roll, the counter roll forming a first press nip with the suction roll and a second extended press nip with the shoe press roll. A felt runs through the two press nips together with the paper web and then brings along the paper web to a Yankee cylinder, to which the paper web is transferred when the felt and the paper web pass round a transfer roll, which forms a non-compressing nip with the Yankee cylinder. Suction zones for dewatering the felt are available before and after the first press nip, the suction zone before the press nip being located inside the suction roll while the suction zone after the press nip is located in a side loop, in which the felt runs alone to meet again the paper web at the entry of the second press nip. Such a paper machine is inconvenient since the paper web is rewet by the wet felt before it reaches the Yankee cylinder.
U.S. Pat. No. 5,393,384 discloses a paper machine for producing a tissue web, which in the embodiment according to FIG. 6 comprises a non-compressible, water-impermeable belt, the underside of which conducts a paper web through a shoe press nip and from there to a Yankee cylinder, via a transfer roll which forms a nip with the Yankee cylinder. This impermeable belt has a smooth web-carrying surface which makes an adhesive water film form thereon as the belt passes through the press nip together with a press felt which has a non-smooth surface in contact with the paper web. As is known, a Yankee cylinder has a smooth surface. Since both the Yankee cylinder and the impermeable belt have smooth surfaces which the paper web is intended to contact, there is a risk that the paper web continues to adhere to the smooth surface of the impermeable belt after having passed the nip adjacent to the Yankee cylinder instead of being transferred, as desired, to the smooth surface of the dryer cylinder. Not even if large amounts of adhesive are applied to the circumferential surface of the dryer cylinder will it be possible to ensure that the paper web adhere to the Yankee cylinder.
The invention starts from the technique described above and its object is to provide a transfer belt, by means of which the soft tissue web can be transferred to a Yankee cylinder in a reliable or more reliable manner.
U.S. Pat. No. 5,298,124 produces an excellent presentation of the tasks which a transfer belt in corporation with a press felt should perform in a satisfactory manner, as well as the properties and construction of such transfer belts which were then disclosed in U.S. Pat. Nos. 4,483,745; 4,976,821; 4,500,586; 5,002,638; 4,529,643 and CA-A-1,188,556.
The critical tasks, according to U.S. Pat. No. 5,298,124, of a transfer belt intended for cooperation with a press fabric comprise a) removing the paper wet from the press fabric without causing instability problems; b) cooperating with the press fabric in one or more nips to ensure optimal dewatering and a high quality of the paper web; and c) transferring the paper web in a closed draw from a press in the press section to a paper-receiving wire or belt in the subsequent press (presses) in the press section, or to a pick-up wire in the dryer section.
U.S. Pat. No. 5,298,124 suggests a transfer belt for the press section in a paper machine having a specific design, and such a transfer belt is shown and described for operation in three paper machines with different press sections, which all comprise a shoe press nip, the belt transferring the paper web from the press section to a dryer fabric which brings the transferred web to a dryer cylinder.
The transfer belt disclosed in U.S. Pat. No. 5,298,124 for the press section of a paper machine has a web-contacting surface which is essentially impermeable to water and air and which has a pressure-responsive microscale topography. Under the action of the pressure in a press nip of the press section, the transfer belt is compressed such that the microscale roughness of said surface decreases, making the surface much smoother and allowing a thin, continuous film of water to be built up between the paper web and said surface. The thin, continuous film of water provides much stronger adhesive forces between the paper web and the transfer belt than between the paper web and the press fabric, so that the paper web may reliably follow the transfer belt as the paper web leaves the press nip. In this connection, the transfer belt expands in the direction of thickness and approaches its non-compressed state such that the film of liquid on said web-contacting surface breaks up.
According to the invention, it has surprisingly been found that a belt of the type according to U.S. Pat. No. 5,298,124 is well suited also for the transfer of a soft tissue web in a closed draw from a shoe press nip in the press section to a Yankee cylinder in a the dryer section of a soft tissue paper machine. As is known, a shoe press nip results in a great degree of dewatering.
The essentially impermeable and elastically compressible transfer belt according to the invention thus consists of an endless carrier, alternatively comprising a joinable seam, with a polymer coating on its web-contacting surface having a hardness of 50-97 Shore A, the polymer coating having a non-compressed degree of roughness of Rz=2-80 xcexcm, measured according to ISO 4287, Part I, and is compressible to a lower degree of roughness of Rz=0-20 xcexcm when a linear load of 20-200 kN/m is applied to the essentially impermeable belt and is capable of being reset to its non-compressed degree of roughness when the pressure acting on the essentially impermeable belt ceases. The Rz value is the ten point height that in said ISO standard is defined as the average distance between the five highest crests and the five deepest troughs of the reference length measured from a line which is parallel to the centre line and does not intersect the surface profile. Preferably the essentially impermeable transfer belt has an air permeability of less than 6 m3/m2/min measured according to xe2x80x9cStandard Test Method for Air Permeability of Testing Fabrics, ASTM D 737-75, American Society of Testing and Materialsxe2x80x9d.
It is surprising that such a transfer belt, which according to U.S. Pat. No. 5,298,124 is intended for pressing in a press section and usable for transfer of a paper web from the press section to a dryer fabric, is usable in a very advantageous manner to transfer a soft tissue web from a shoe press nip directly to a Yankee cylinder. In a Yankee cylinder, the conditions are, in fact and as is known, quite different from those in a press nip. In a Yankee cylinder, there is no pressing of the soft tissue web for direct dewatering, but it is instead a matter of supporting the soft tissue web against the outer surface of the Yankee cylinder, such that the fibres of the soft tissue web adhere to the surface of the Yankee cylinder in order to get stuck by burning, thereby obtaining good thermal transmission to the paper web. Precisely this effect is achieved by the inventive transfer belt, which cannot be achieved by a press felt according to DE 195 48 747 owing to the above-mentioned rewetting of the paper web after the last press nip in the press section, which prevents good adhesion, and cannot be achieved or is achieved to a substantially smaller extent by a transfer belt according to U.S. Pat. No. 5,393,384, for the reason described above. The compressibility of the inventive transfer belt results in a lower specific pressure in the adhesion point, which in turn entails increased runnability, i.e. a higher production rate. Moreover, this property causes an increased evaporation of water from the soft tissue web, i.e. quicker drying of the soft tissue web on the Yankee cylinder, which also contributes to a higher efficiency of the process. The increased efficiency can be used either as a higher machine speed or as a reduced consumption of energy while retaining the production volume.